Effect of Different Instrumentation Techniques on Vertical Root Fracture Resistance of Endodontically Treated Teeth

Statement of the Problem Vertical root fractures are catastrophic events that often result in tooth extraction. Many contributing factor are associated with increasing incidence of vertical root fracture. Root canal preparation is one of the predisposing factors which can increase the root susceptibility to vertical fracture. Purpose The aim of this study was to compare the effects of three different instrumentation techniques on vertical root fracture resistance of endodontically treated teeth. Materials and Methods In this study, 120 freshly extracted mandibular premolar teeth of similar dimensions were decoronated and randomly divided into control (n=30), nickel-titanium hand K-file (HF, n=30), BioRaCe rotary file (BR, n=30), and WaveOne reciprocating single-file (WO, n=30) groups. After cleaning and shaping the root canals, AH26 was used as canal sealer, and obturation was completed using the continuous wave technique. The root canals were embedded vertically in standardised autopolymerising acrylic resin blocks, and subjected to a vertical load to cause vertical root fracture. The forces required to induce fractures were measured using a universal testing machine. ANOVA and Tukey’s post-hoc test were used to analyse the data. Results All experimental groups showed statistically significant reductions in fracture resistance as compared with the control group. There was a statistically significant difference between the HF and BR groups. The WO group did not differ significantly from the HF group or the BR group. Conclusion All three instrumentation techniques caused weakening of the structure of the roots, and rendered them susceptible to fracture under lesser load than unprepared roots. The fracture resistance of roots prepared with the single-file reciprocating technique was similar to that of those prepared with NiTi hand and rotary instrumentation techniques.


Introduction
for extraction of endodontically One of the reasons ertical root V . is vertical root fracture treated teeth fracture (VRF) is a longitudinally oriented fracture, The [1] . extending from the root canal to periodontium  [4] criticised to cause VRF because of its potential to weaken the tooth structure and predispose the tooth to fracture. [5] In a study by Adorno et al. (2013), crack initiation was significantly related to preparation, while root canal filling techniques were significantly associated with the propagation of these cracks. Rotary instrumentation has been associated with more cracks compared with hand instrumentation. [6][7][8] These cracks can gradually degenerate into VRFs. Lam et al. (2005) found no increase in fracture susceptibility when comparing the rotary and hand instrumentation. [9] Advances in nickel-titanium (NiTi) rotary instruments have led to the introduction of canal instrumentation systems with different file designs, metallurgical alloys, and rotational motions. Despite having several advantages compared with the traditional hand instruments, these files are associated with high stress generation within the root canals. [6,8,10] Different NiTi instrument designs are associated with different levels of stress and resistance of roots to fractures. [11][12] The single-file reciprocating WaveOne (Dentsply-Maillefer; Ballaigues, Switzerland), an improvement in gradual shaping with multiple instruments, seems to be an attractive option even for novice operators. [13] It is also claimed to be cost-effective and less time-consuming, due to the reduced number of files used compared with the multi-instrument rotary canal preparation techniques. [14] WaveOne is a single-file reciprocating system featuring variable design along with its length, reciprocating motion, and the unique NiTi alloy called 'M-Wire'. The file cuts counter clockwise (CCW), and its angle of rotation is five times greater in the CCW direction than in the clockwise (CW) direction, which is designed to enhance the resistance of the file to fracture.
CW movement of the file disengages the instrument from dentin, relieves the stress as it progresses into the canal, and thereby decreases the chance of taper lock. [15] A study by Burklein et al. (2013) compared root canal preparation performed with single-file reciprocating systems with that performed with sequential full rotational files. They showed that defects occurred independently of the instrumentation technique, but reciprocating instruments created more cracks in the apical third of canals. [16] In another study, Ashwinkumar et al. (2013) observed more micro-cracks associated with the rotary ProTaper Universal file (Dentsply-Maillefer) than with reciprocating WaveOne or ProTaper hand fil-es, and no micro-cracks with NiTi hand K-files. [7] This study aimed to compare the differences in fracture resistance of the roots prepared with NiTi hand K-file (HF, Dentsply-Maillefer; Ballaigues, Switzerland), BioRaCe rotary file (BR, FKG Dentaire; La-Chaux-de-Fonds, Switzerland), and large WaveOne reciprocating single-file (WO, Dentsply-Maillefer), in addition to unprepared root canals as control group.
Complete canal preparation with a single-file instrument might be assumed to generate more stresses, since only a single file performs the entire enlargement of the canal, which can increase the incidence of dentinal defects, and reduce resistance to VRF. The null hypothesis tested was that there would be no difference between the four groups.

Tooth selection
Prior to conducting the study, the research protocol was

Instrumentation
Tooth working length (WL) was determined by subtracting 1 mm from the length at which a size 10 K-file became visible at the root apex. The reference point was the flat coronal surface of each root. An operator, experienced in rotary and reciprocating systems, prepared all the root canals. Root canals in each group were prepared as follows:

Control group
Root canals were irrigated with 1% sodium hypochlorite solution without instrumentation or obturation.
Step back technique using NiTi hand K-files (HF) ISO 0.02 taper NiTi hand K-files (Dentsply-Maillefer) were used in the ISO size sequence 15,20,25,30,35 and size 40 as the master apical file (MAF) to size 60 as the last file used, with 1 mm incremental reduction from the WL determined by the step back technique. Instruments were regularly cleaned, and root canals were irrigated copiously with 1% sodium hypochlorite solution, followed by recapitulation with the MAF at WL after each step back. Each set of files was used to prepare four root canals.

BioRaCe rotary NiTi files (BR)
In accordance with the manufacturer's recommenda- Newton. The maximum load during each test was defined as the fracture load. After each fracture test, the roots were dyed with 2% methylene blue dye solution and viewed under magnification, to confirm the fracture and determine the pattern of the fracture lines to be buccolingually, mesiodistal, or compound.

Statistical analysis
Data were analysed using the Kolmogorov-Smirnov normality test and ANOVA. Once a significant difference in score was found (p< 0.05), Tukey's post-hoc test was used to determine significant differences in average scores between specific groups. All statistical analyses were performed at a 95% level of confidence.

Results
No instrument fractured during instrumentation. Table 1 shows the mean load at fracture in each group.

Discussion
The [ 10,17] File design can result in dentinal defects and reduce the fracture resistance of roots. [11] Stiffer files generate higher stress concentration. Stiffness is related to size, taper, cross-section, method of manufacturing, and the material out of which the instrument is made [18]. M-Wire is a more flexible type of conventional NiTi from which WaveOne instruments are made. [19] Concerning the designs of the files used in this study, WaveOne files feature a modified convex triangle with radial lands from D1 to D8, and a convex triangle from D9 to D16, while, the BioRaCe has reverse-acting cutting edges with a simple triangular cross section. Evidently, instruments with a triangular cross-section have more even stress distributions along their length, and lower stress concentrations, than the instruments with rectangular cross-sectional designs, which can create higher stress differentials during simulated canal shaping. [18] Canal diameter can also affect the root resistance to vertical fracture. [12] It is not surprising that remov-ing more dentin reduces the fracture resistance of roots. [20][21] Excessive dentin removal would unnecessarily weaken and compromise the structural integrity of roots; thus it should be avoided. Additionally, more craze lines are found in areas with more root structure removed, [20] and the larger the diameter of the canal, the less the resistance to fracture. [22] In this study, the apical diameters of all experimental groups were similar and there was no significant difference despite Wave-One having a greater taper than BioRaCe (0.04 for the final finishing files) and hand NiTi K-files (0.02). [19] This could be because of the difference in their rotation- The wide variation in load at fracture observed in the control group (Figure 1)

Conclusion
Root canal treatment weakens roots, and in this study, instrumentation with the single-file reciprocating technique was associated with resistance to fracture comparable with the roots prepared with NiTi hand or rotary instruments.